British patent specification No. 1,127,050 (claiming priority from German patent application No. Sch. 36854 IVb/12o dated Apr. 9th, 1965) (to Schering) discloses substituted phenylcarbamates of the general formula A ##STR2## in which R.sub.1 represents an alkyl group, a cycloalkyl group or an aryl group, which aryl group may be substituted by a halogen atom and/or an alkyl group and/or a trifluoromethyl group, R.sub.2 represents a hydrogen atom or an alkyl group, or R.sub.1 and R.sub.2 together with the nitrogen atom may form a heterocyclic ring that may contain further nitrogen and/or oxygen atoms and R.sub.3 represents an alkyl, alkenyl or alkynyl group that may be substituted by a terminal halogen atom, which compounds are useful for selectively combating weeds in crop areas. The herbicidal activity is observed in both pre-emergence and post-emergence treatments, thus allowing preparations containing said compounds A to be applied by whichever method is desired. The herbicidal activity is also observed when compounds A are applied on the leaves of established weeds, and the compounds are especially useful in combating weeds in beet fields, especially as post-emergence herbicides in sugar beet fields.
An especially preferred compound of the general formula A is methyl(3-(3-tolyl-carbamoyloxy)phenyl)carbamate which has later been given the common name phenmedipham. Another especially preferred compound is ethyl(3-phenyl-carbamoyloxyphenyl)carbamate which has later been given the common name desmedipham. Methyl(3-(3-tolyl-carbamoyloxy)phenyl)carbamate is commercially available under the trade mark Betanal and is used especially as post-emergence herbicide for the weeding of beet crops, in particular sugar beet, at an application rate of 1 kg a.i./200-300 liter/ha. It has been reported (Pesticide Manual, A World Compendium, The British Crop Protection Council, 6th Edition, 1979) that the compound acts through the leaves with little action via the soil and roots. In the soil there was, in three months, a 71-86% degradation of the a.i. (active ingredient) detected one day after treatment.
According to the British patent specification, compounds A may be prepared, e.g., by the following processes:
When R.sub.2 represents a hydrogen atom, by reacting N-hydroxyphenyl carbamates of the general formula ##STR3## with
(a) isocyanates of the general formula EQU R.sub.1 --N.dbd.C.dbd.O
in the presence of a catalyst, suitably an organic base, preferably triethylamine, or with
(b) carbamic acid chlorides of the general formula ##STR4## in the presence of an acid acceptor, suitably an inorganic or organic base, preferably pyridine, or, when R.sub.2 represents alkyl, or is a member of a heterocyclic ring, the indicated N-hydroxyphenyl carbamates are reacted as indicated with the indicated carbamic acid chlorides, R.sub.1 and R.sub.3 in the above formulae having the definitions given above and R.sub.2 having the corresponding definition in each case.
According to the above-mentioned British patent specification, both reaction (a) and reaction (b) are carried out in organic solvents such as tetrahydrofuran or anhydrous pyridine in order to avoid the influence of water on the isocyanate.
British Pat. No. 1,173,753 (to Schering) discloses herbicidally active compounds having a formula closely related to formula A above, i.e., compounds of the general formula B ##STR5## in which R.sub.A represents an alkyl or alkenyl group of at the most 5 carbon atoms, an alkynyl group of 3 or 4 carbon atoms, a cycloalkyl group of 3-6 carbon atoms, a phenyl group, a benzyl group or a methoxyethyl group, R.sub.B represents an alkyl group of 1-8 carbon atoms, an alkenyl group of 2-4 carbon atoms, a cycloalkyl group of 3-6 carbon atoms, an optionally by halogen substituted phenyl group, a methoxyphenyl group, a methylphenyl group or a chloroethyl group, R.sub.C represents hydrogen or an alkyl group of 1-3 carbon atoms, or R.sub.B and R.sub.C together with the nitrogen atom to which they are attached form a piperidino group, and R.sub.D represents hydrogen or methyl, and X represents oxygen or sulphur, except compounds used as active components in the herbicidal preparation according to British Pat. No. 1,127,050.
According to the examples in British patent specification No. 1,173,753, the preparation of the 3-(carbamoyloxy)carbanilates is performed in organic solvents such as ether, ethyl acetate, benzene, etc.
Tetrahydrofuran, pyridine and many other organic solvents are known to be hazardous to the health. The use of organic solvents demands separation thereof from the effluents, and from an economical point of view it is necessary to re-use organic solvents.
The above-mentioned British patent specification No. 1,127,050 also discloses that the N-hydroxyphenyl carbamates required as starting products may be prepared by N-acylation of 3-aminophenol with the appropriate chloroformic acid esters, for example, in a mixture of ethyl acetate and water in the presence of magnesium oxide. The effect of ethyl acetate is to dissolve the carbamate as it is formed, and the water dissolves the magnesium chloride formed. 3-Aminophenol is not dissolved by ethyl acetate, and the reaction medium thus contains 3-aminophenol and magnesium oxide suspended therein. Excess magnesium oxide can be dissolved in dilute HCl, thus yielding two liquid phases. The isolation and purification of the product is rather complicated due to the character of the two-phase medium, the organic medium comprising the N-hydroxyphenyl carbamate dissolved therein. The phases are separated, and the organic phase is washed neutral with water and then with dilute potassium bicarbonate solution. After drying and evaporation of the organic phase, the crude product is redissolved in ether and precipitated with light petroleum. The yield of the N-hydroxyphenyl carbamate is stated to be 83%, based on the starting material 3-aminophenol. The loss is probably due to the loss of 3-aminophenol in the form of hydrochloride. Due to the contents of MgCl.sub.2, the aqueous phase cannot be used in the next batch and hence, it is necessary to extract 3-aminophenol which, however, is only slightly soluble in common organic water-immiscible solvents.
Hence, the known methods for the preparation of the starting materials are disadvantageous from an economic point of view as a loss of starting material of 17% is unsatisfying, and the isolation and purification is complicated, and the process for the preparation of the end products, the substituted phenyl carbamates, is disadvantageous in being carried out in solvents which are hazardous to the health.